The transient or pull-in period of Phase-Locked Loop (PLL) circuits at frequency changes is inversely proportional to the limit frequency fg of the loop. Frequency fg is that frequency where the amplification factor of the open circuit is 1, i.e. 0 dB gain. The higher the limit frequency, the faster a PLL circuit will stabilize.
However, in stabilized condition, the behavior of a PLL circuit with smaller limit frequencies is preferable. For instance, the lower the limit frequency, the higher the attenuation of a reference frequency supplied to the phase detector or comparator circuit. Especially for PLL circuits for radio receivers, where the reference frequency deviates significantly from the oscillator frequency, disturbances in the tuning voltage due to the reference frequency are noticeable.
Quick stabilization and low disturbance are, therefore, opposing requirements. Thus, a compromise has to be found for generally utilized PLL circuits. The need for a short transient or pull-in period is particularly strong for receivers that are briefly toggled between an initially received frequency and a second frequency to be checked. Such receivers are used in the ARI (registered trademark of Blaupunkt Werke) traffic information system and in the RDS system defined by the European Broadcasting Union. In car radios, this may happen in order to check whether stations with other transmitting frequencies carry either the same program, or traffic announcements valid for the same geographic area, albeit with reception at a higher field strength. This switching-over for checking is so short, that, depending on other measures used, minimal disturbance to the reception occurs.
Known circuits for switching-over, or toggling the limit frequency fundamentally operate by toggling loop amplification, for instance by toggling the output currents of the PLL circuit or by switching between resistors in the loop filter. However, by doing so, the charactistic curve, that is, the slope of the loop amplification with respect to the frequency in the range of the limit frequency, is steepened, which is associated with poorer stability of the PLL circuit.